The effect of yeast storage conditions on subsequent fermentations.

Abstract
It is well known that control of fermentation rate contributes to final beer quality. Though fermentation conditions such as pitching rate, temperature and wort nutrient levels are controlled, there are still inconsistencies in fermentation rates. Recent work has suggested that the history of the yeast is important, particularly cellular glycogen levels. Glycogen acts as the energy supply during the lag phase of fermentation when there is a high demand for ATP for synthesis of sterols and fatty acids, i.e. cell membrane components. Early in fermentation when glucose has not entered the cells, oxygen, essential for lipid synthesis, has rapidly entered the cells. Therefore to synthesise lipid, glycogen is mobilised. Glycogen use and lipid synthesis are both rapid, i.e. within six hours of pitch. Later in fermentation glycogen reserves are built up again. Amounts stored will affect fermentation potential at repitch. The relationship between levels of this storage carbohydrate and either fermentation rate or yeast storage conditions have been studied. Lager yeast with differing glycogen levels (28%, 15% and 9% of yeast dry weight) was used to ferment 16 degrees Plato wort containing 30% corn adjunct. Wort oxygen level was 16 ppm and fermentation temperature was 14.5 degrees C. Pitching rate was 0.3 g wet cells per 100 ml wort. This was corrected for yeast viability measured by methylene blue staining. Yeast with the highest level of glycogen gave the fastest fermentation and more yeast growth. Low yeast glycogen levels could be compensated by increasing pitching rates When yeast was stored at 15 degrees C for 48 hrs glycogen dropped to around 50% of the original level, whereas at 4 degrees C this figure was nearer 90%. With greater oxygen access glycogen levels dropped more rapidly although lipid levels are consistent - perhaps other nutrients are needed for lipid synthesis. Good yeast handling includes avoiding access of oxygen to the slurry and cooling the yeast to 4 to 6 degrees C as soon as possible. Pitching rates may be adjusted to compensate for low glycogen levels. A rapid method to determine glycogen levels is available, though is not standardised for yeast at all stages of the brewery cycle. Alternatively microscopic examination of yeast cells stained with Lugol's iodine may be useful.Keywords: brewing fermentation glycogen lipid storage yeast